1. Field of the Invention
The present invention relates to a visual display apparatus, for example, a head- or face-mounted visual display apparatus that can be retained on the observer's head or face.
2. Background of Related Art
For a head-mounted visual display apparatus, it is important to minimize the overall size and weight thereof in order to make the observer feel comfortable when wearing it. An essential factor in determining the overall size of the apparatus is the layout of the optical system.
FIG. 24 shows the optical system of one conventional head-mounted visual display apparatus [Japanese Patent Application Laid-Open (KOKAI) No. 3-101709 (1991)]. In this visual display apparatus, an image that is displayed by a two-dimensional image display device is transmitted as an aerial image by a relay optical system including a positive lens, and the a&rial image is projected into an observer's eyeball as an enlarged image by an ocular optical system formed from a concave reflecting mirror. A direct-vision layout has also been known in which an enlarged image of a two-dimensional image display device is observed directly through a convex lens. With these conventional layouts, the amount to which the apparatus projects from the observer's face is unfavorably large. Further, it is necessary in order to provide a wide view angle for observation to use a positive lens having a large diameter and a large two-dimensional image display device. Accordingly, the apparatus inevitably becomes increasingly large in size and heavy in weight.
To enable the observer to use the visual display apparatus for a long time without fatigue and to attach and detach the apparatus with ease, it is preferable to adopt an arrangement in which a short and lightweight ocular optical system is disposed immediately in front of the observer's eyeball. With this arrangement, a two-dimensional image display device, an illuminating optical system, etc. can be disposed so that the apparatus projects forwardly from the observer's head to a minimal extent. Thus, it is possible to reduce the amount of projection of the apparatus and also the weight thereof.
Next, it is necessary to ensure a wide angle of view in order to enhance the feeling of being at the actual spot which is given to the observer when viewing the displayed image. In particular, the stereoscopic effect of the image presented is determined by the angle at which the image is presented (see The Journal of the Institute of Television Engineers of Japan Vol. 45, No. 12, pp. 1589-1596 (1991)). The next matter of great concern is how to realize an optical system which provides a wide angle of view and high resolution. It is known that it is necessary in order to present a stereoscopic and powerful image to the observer to ensure a viewing angle of 40.degree. (.+-.20.degree.) or more in the horizontal direction, and that the stereoscopic and other effects are saturated in the vicinity of 120.degree. (.+-.60.degree.). In other words, it is preferable to select an angle of view which is not smaller than 40.degree. and which is as close to 120.degree. as possible.
Incidentally, U.S. Pat. No. Re. 27,356 discloses an ocular optical system which, as shown in FIG. 25, uses a semitransparent concave mirror 6 and a semitransparent plane mirror 16 to project an object surface 62 in at a distance. With this arrangement, the optical path from the image display device 62 to the observer's eyeball position 66 can be shortened by repeated reflection of light rays, and it is possible to reduce the amount to which the visual display apparatus projects forwardly from the observer's head.
However, since the concave mirror 6 produces a strong curvature of field along the surface thereof because of its nature, if a planar two-dimensional image display device is disposed at the focal point of the concave mirror 6, the resulting observation image surface is curved, so that it is impossible to obtain an image for observation which is clear as far as the edges of the visual field. Therefore, it is necessary to correct the field curvature by curving the object surface 62.
In general, when a CRT is used as a display device, a curved display surface can be realized with relative ease. However, CRTs are heavier in weight than liquid crystal devices. Further, in order to provide the observer with an image which is clear as far as the edges of visual field at a wide angle of view, it is necessary to use a two-dimensional image display device which has a large size and a high pixel density, and it is preferable to use a thin and lightweight image display device, e.g., an LCD. However, it is extremely difficult to curve the display surface of a planar image display device, e.g., an LCD, on account of the structure thereof.
Accordingly, even if the above-described ocular optical system is used, it has heretofore been difficult to realize a compact and lightweight visual display apparatus which enables observation of an image which is clear as far as the edges the of visual field at a wide viewing angle.